1. Field of the Invention
This invention relates to an insulating spacer which is disposed between two members, such as electrodes, differing in electric potential in an electric apparatus, equipment, etc., to maintain an insulation distance between the two members.
2. Description of the Prior Art
FIG. 1 shows a sectional view of an insulating spacer according to the prior art. In the figure, reference numbers 1 and 2 denote a higher- and a lower-potential electrode, as two members differing in electric potential, and reference number 3 denotes platelike insulating barriers disposed between the electrodes 1 and 2 to prevent flashover between the electrodes. Reference number 4 denotes unit spacers made of an insulating solid material and having a bar shape with a tetragonal cross section, three such unit spacers being disposed between the higher- and lower-potential electrodes 1 and 2 to maintain an insulation distance between the electrodes and to clamp each of the insulating barriers 3 between the unit spacers 4, thereby maintaining the barriers in position.
The unit spacers 4 and insulating barriers 3 in practical use in an electric apparatus (not shown) or the like lack uniformity in their dimensions or have irregularities in their surfaces. Therefore, it is not always the case that the space between the electrodes 1 and 2, in which the unit spacers 4 are disposed, is filled completely with the insulating solid material as shown in FIG. 1. Namely, a gap 5 may in some cases be generated in the space, as for instance illustrated in FIG. 2. The gap 5 is filled with an ambient insulating medium such as a gas (air, sulfur hexafluoride, etc.) and an insulating oil (neither shown). Assuming that the specific dielectric constant of the insulating medium in the gap 5 is .epsilon..sub.2, the specific dielectric constants of the insulating barriers 3 and the unit spacers 4 are equally .epsilon..sub.2, the length of the gap 5 generated in the space between the electrodes 1 and 2 is d.sub.1, while the total dimension of solid insulator portions is d.sub.2, and the potential difference between the electrodes 1 and 2 is V, then the electric field strength Eg in the gap 5 is ##EQU1## On the other hand, the average electric field strength E.sub.0 between the electrodes 1 and 2 is ##EQU2## In general, an insulating solid material (inclusive of one which is impregnated with an insulating medium) in most cases has a higher specific dielectric constant than that of an insulating medium. By way of example, here, a case where .epsilon..sub.1 =1 and .epsilon..sub.2 =3 will be dealt with. When d.sub.1 /d.sub.2 is varied, the ratio Eg/E.sub.0 calculated from the equations (1) and (2) and taken as field concentration factor is varied as represented by the graph shown in FIG. 3. When the length d.sub.1 of the gap 5 is small, the field concentration factor is 3 at maximum, that is, the field strength Eg in the gap 5 reaches 3 times the average field strength E.sub.0. Thus, the portion of the gap 5 is exposed to very severe conditions on an insulation basis and, if Eg exceeds the dielectric strength of the insulating medium in that portion, a partial discharge might result. Therefore, careful consideration should be given to the field strength Eg in the gap 5 in designing the electric apparatus. While the above description has been based on the case of .epsilon..sub.2 /.epsilon..sub.1 =3, the field concentration factor Eg/E.sub.0 will be further greater where the ratio .epsilon..sub.2 /.epsilon..sub.1 is more than 3, so that special care should be taken of selection of the combination of the insulating medium with the solid insulating material.
The conventional insulating spacers, constructed as above, have had the possibility of a gap being generated to cause a local concentration of electric field on the gap. The conventional insulating spacers have therefore been limited in selection of the insulating solid material, constituting the insulating spacers, and the material for the insulating medium surrounding the spacers. In some cases, it has been necessary to take such countermeasure as enlarging the distance between the electrodes or the like members to lower the average field strength therebetween.